A seismic velocity profile in a subsurface formation may be obtained employing the so-called check shot (also known as “well shoot” or “velocity survey”) technique. The check shot technique is described in, for instance, on pages 167 and 168 of “Well Logging in non-technical language,” by David Earl Johnson and Kathryne E. Pile, published 2002 by PennWell Books, ISBN 0878148256.
A conventional check shot technique includes generating multiple shots from a seismic source located in a selected position, usually close to a wellhead. Shots are detected at different locations along a bore hole extending below the wellhead, by a receiver that is moved along the bore hole.
Alternatively, a set comprising a plurality of receivers may be located in the bore hole, in which case only a single shot would be required.
A complex overburden may lead to erroneous velocity profiles when conventional check-shots and zero-offset vertical seismic profiles (VSP) are used.
When there is a complex transmission medium between the set of seismic sources and the region of interest, such as for example a complex overburden, the extraction of velocity profiles from check shots is a challenging task. First arrival waveforms may be distorted and difficult to pick as a result of the complex transmission medium, and the fastest arrival may not even come along the shortest path between the source and the receiver, thus making it difficult to relate arrival time to velocity.
This could be alleviated by placing the source in the bore hole, or well, so that the travel-path of the first arrival is short and close to a straight line between the source and the receiver. In practice, however, it is cumbersome to place and operate a physical source downhole.
Another difficulty with conventional check shots is that they measure only P-wave velocity from first arrivals. Shear waves, even if emitted from the surface source or generated by P-S conversions in the overburden, arrive later, and are difficult to unravel and pick. Hence a shear wave check shot is difficult, if not impossible, to obtain using conventional check shot techniques.